I just learned of a peculiar kind of tumor called a teratoma. It forms in the testicles or ovaries due to the presence of pluripotent cells in those regions. These cells, basically, start dividing and forming new organs, with the DNA of the progenitor cell. A mature, differentiated teratoma can contain, teeth, hair, skin and even a tongue. Here's an X-ray of a woman's pelvis where a tumor formed that contains mature teeth. These tumors occur in nature on a regular basis, occuring in newborns about 1 in 20,000 births, in adult males about 1 in 700,000, and in adult females about 1 in 300,000.

This is also what happens if embryonic stem cells are allowed to differentiate unchecked within an adult body. For a long time, I figured that the environmental cues of the body would be enought to coax ESC's into differentiating correctly. Apparently that's not the case.

I don't think anyone's tried injection of cloned ESC's, which would be more likely to accept differentiation signals from the host, and probably dosages would have to be very low to prevent clustering. If there were some way to create an electrostatic colloid or suspension, such that no two cells were adjacent, then the chances of feeback cues would even be lower. I suspect that feedback cues are (partly) responsible for the formation of teratoma's. This is still a line of experimentation that deserves further inquiry.